A measuring device for determining the concentrations of gases by infrared absorption, which has two radiation sources and two radiation detectors, is known from DE 197 13 928 C1. A first radiation detector is used as a measuring detector and a second radiation detector as a reference detector. The wavelength range of the radiation detector is coordinated with the absorption of the radiation by the substance, whose concentration is to be measured. The reference detector measures the radiation in a range that is not affected by this substance. The reference signal provided by the reference detector is used to compensate variations of the incident radiation capacity.
To obtain a good measurement result, it is desirable that the radiation guide means of the measuring device is designed such that the radiation detector is reached by the highest radiation intensity possible, so that a high signal-to-noise ratio can be obtained. The radiation guide means may have means for this purpose, to ensure that the radiation is focused to the radiation detector. However, sharp imaging of the radiation on the detector has the drawback that the accuracy of the measurement result depends very strongly on the adjustment of the radiation guide means. To avoid the undesired effects of maladjustment, provisions may be made for producing an enlargement of the radiation spot in an area in front of or behind the radiation detector by focusing the radiation. However, undesired shadowing of the radiation, which may lead to considerable errors of measurement, are associated with such a “defocusing.”
Non-imaging concentrators are arranged in front of the radiation detectors in the device known from DE 197 13 928 C1. These shall bundle the radiation incident in parallel and generate a uniform radiation distribution in the plane of the radiation detectors. However, the drawback is that a shift of the intensity distribution takes place in case of maladjustment, and different intensity reductions generate varying signal ratios, which in turn lead to a drift.